Power steering systems of the kind described herein are shown in German patent publications DE-AS No. 12 81 872 and DE-OS No. 19 40 490. In such systems, piston valves are used synchronized with a servomotor piston wherein a steering nut carried by the servomotor piston has a guide element elongated parallel to axis of the nut and servomotor piston engaged in a groove of the piston valve so that rotation of the nut by means of a steering spindle initially actuates the piston valve in a selected direction for pressurizing one chamber or the other of a servomotor. As the piston moves, the nut moves axially with it and the guide element rides in the groove of the piston valve in an axial longitudinal direction. The effect is to synchronize the steering angle of the nut in relation to the housing which contains the servomotor piston, steering system, etc., so as to ensure the actuated position of the piston valve with the nut moving.
In such constructions, moving in straight line, there is no independent shifting effect of the guide element acting on the piston valve since the moving path of the guide element is parallel to the axis of the nut and the servomotor piston.
Centering of the valve piston and nut is effected by springs at the ends of the valve piston, for example, as shown in DE OS No. 18 17 940; German publication DE OS No. 18 17 940.
In the prior art, it has been customary to provide means for returning the vehicle wheels to a straight ahead position after steering by means of axial geometry, e.g., toe-in, and other mechanical arrangements. However, there is a disadvantage in such mechanical arrangements, in that the wheel restoring force must be supplied as energy to the restoring mechanism and, accordingly, higher steering forces are required.